A. Field of the Invention
This invention relates to a novel process for continuously producing shaped articles of oxadiazole polymers from certain monomer solutions. As used herein, the term "oxadiazole polymer" means a fiber-forming polymer consisting essentially of recurring units of the formula -- AR -- X --, where Ar is a divalent aromatic radical and X is a radical selected from the group consisting of ##STR1## alkyl, with the proviso that in at least 20% of said units X is ##STR2## the term "monomers" means monomers from which oxadiazole polymers are prepared; and the term "monomer solution" means an oleum, chlorosulfonic acid and/or polyphosphoric acid solution of monomers.
Inherent viscosity (.eta..sub.inh) values, when given herein, are measured at 25.degree. C. and are defined by the following equation: ##EQU1## where C represents a solids concentration of 0.2 gram of polymer per 100 ml of concentrated sulfuric acid as solvent and RV represents relative viscosity. The relative viscosity (RV) is determined by dividing the flow time in a capillary viscometer of a dilute polymer solution by the flow time for the pure solvent. The solids concentration of the dope is calculated and then a portion or sample of this dope is diluted with an appropriate amount of concentrated sulfuric acid to obtain a solution having a solids concentration of 0.2 g. of polymer per 100 ml of concentrated sulfuric acid. Brookfield viscosity values, when given herein, are measured at 25.degree. C. by conventional techniques.
B. Description of the Prior Art
Processes for preparing fibers of oxadiazole polymer from monomer solutions are taught in British Pat. Nos. 1,407,439 and 1,417,568 and U.S. Pat. No. 3,886,251. These processes involve heating a monomer solution in a kettle-type reactor at a temperature of from 140.degree.-150.degree. C. for from 4 to 6 hours in order to effect polymerization of the monomers and provide a dope (polymer solution). The dope is diluted with concentrated sulfuric acid to reduce its viscosity to an acceptable level for extrusion and then degassed to prevent extrusion difficulties and product deficiencies. The diluted and degassed dope is then cooled to a temperature of less than about 100.degree. C. and extruded from the kettle-type reactor or other similar reservoir through a heated spinneret into an aqueous medium to form fiber.
The above-described processes have several serious drawbacks from the standpoint of commercial scale operations. A major disadvantage of these processes is that the viscosity of dopes which can be extruded by utilizing the process is limited to about 30,000 poise which in turn limits the solids concentration of the dopes. The term "solids concentration" as used herein means that portion of the dope, expressed as a weight percent, which would be expected to precipitate as oxadiazole polymer when the dope is combined with an aqueous medium, assuming that the monomers used in preparing the dope reacted stoichiometrically. Naturally, for commercial operations it is desirable to utilize dopes having the highest possible solids concentration so as to attain high productivity and reduce the cost of solvent and solvent recovery. Attempts to utilize dopes of higher viscosity in this process have not proven successful. While the apparent viscosity of the dope can be reduced by elevating the temperature of the dope, unfortunately, significant degradation of the polymer occurs when the dope is heated and held for any period of time at temperatures above about 140.degree. C. prior to extrusion.
It is an object of the present invention to provide a process for producing shaped articles of oxadiazole polymers from monomer solutions which avoids the problems described above.
Other objects of the invention will become apparent to those skilled in the art from the following detailed description of the invention.